Revert "[deserialization] Deserialize WitnessTables/VTables in SILModule, not in SILLinking."

This reverts commit r14623. I will recommit this when I reenable deserialization
of witness tables/vtables. This will enable the deserialization of SILFunctions
by default from the stdlib by avoiding the lack of external witness
tables/vtables.

Swift SVN r14786

lib/SILPasses/Link.cpp

@@ -137,4 +137,9 @@ void swift::performSILLinking(SILModule *M, bool LinkAll) {
       }
     }
   }
+
+  if (LinkAll) {
+    SILLoader->getAllVTables();
+    SILLoader->getAllWitnessTables();
+  }
 }